Abstract

The Sierra La Primavera volcanic complex consists of late Pleistocene comenditic lava flows and domes. ash-flow tuff, air-fall pumice, and cold caldera-lake sediments. The earliest lavas were erupted about 120,000 years ago, and were followed approximately 95,000 years ago by the eruption of about 20 km3 of magma as ash flows that form the compositionally-zoned Tala Tuff. Collapse of the roof zone of the magma chamber led to the formation of a shallow 11-km-diameter caldera. It soon filled with water, forming a caldera lake in which sediment began to collect. At about the same time, two central domes erupted through the middle of the lake and a "giant pumice horizon", an important stratigraphic marker, was deposited. Shortly thereafter ring domes erupted along two parallel arcs: one along the northeast portion of the ring fracture, and the other crossing the middle of the lake. All these events occurred during a period of approximately 5,000-10,000 years. Sedimentation continued and a period of volcanic quiescence was marked by the deposition of some 30 m of fine-grained ashy sediments virtually free from pumice lapilli. Approximately 75,000 years ago, a new group of ring domes erupted at the southern margin of the lake. These domes are lapped by only 10-20 m of sediments, as uplift resulting from renewed insurgence of magma brought an end to the lake. This uplift culminated in the eruption, beginning approximately 60,000 years ago, of aphyric lavas along a southern arc. The youngest of these lavas erupted approximately 20,000-30,000 years ago. The four major fault systems in the Sierra La Primavera are related to caldera collapse or to uplift caused by the insurgence of the southern are magma. Steam vents and larga-discharge 65°C hot springs are associated with the faulting. Calculated equilibrium temperatures of the geothermal fluids are ~170°C, but temperatures in excess of 240°C have been encountered in an exploratory drill hole. A seismic survey showed attenuation of both S and P waves within the caldera, P waves attenuated more severely than S waves. The greatest attenuation is associated with an area of steam vents, and the rapid lateral variations in attenuation suggest that they are produced by a shallow geothermal system rather than by underlying magma.